Many mobile devices, such as cellular telephones, personal digital assistants (PDAs), and other handheld computing and communicating devices, rely upon standard energy storage devices, such as battery cells, for providing power on which to operate.
Though disposable battery cells, such as alkaline cells, are a well-known and reliable technology, it is common in such mobile devices to employ rechargeable battery cells. These rechargeable batteries depend on a number of known cell types, including Ni-Cad, Ni-MH, and Li-Ion cells. All these cells are known to those of skill in the art, as are some of their deficiencies.
Although some mobile devices are able to function with standard off-the-shelf rechargeable batteries, many use a specialised rechargeable battery made particularly for that make and model of mobile device. A charging device is necessary in order to recharge the mobile device's battery. Such a charging device may be a dedicated device, or may be integrated into an existing accessory, such as a cradle. The life of the battery can be drastically curtailed by improperly charging, or over discharging the battery.
Over-voltage protection circuits are commonly used to prevent a voltage across an energy storage device, such as a battery, from exceeding a set predetermined, or threshold, voltage. Such an energy storage device can comprise a plurality of energy storage components. Presently, over-voltage protection is typically achieved by connecting resistors in parallel with the energy storage device. In such over-voltage circuits, current continuously flows through the resistors whether the terminal voltage is above or below the set predetermined voltage, resulting in significant wasted power. Such a conventional configuration is illustrated in FIG. 1.
The energy storage devices 102, 104 illustrated in FIG. 1 are super capacitors, showing an example of a particular energy storage device. However, those of skill in the art will appreciate, the energy storage devices can be any suitable device, such as Ni-Cad, Ni-MH, and Li-Ion cells, for example.
FIG. 1 shows a typical over-voltage protection circuit that is well known in the art. In this circuit 100, energy storage devices 102, 104 are connected in series. Each energy storage device has a parasitic internal leakage current. The magnitude of the leakage current may vary over a range of values, even among energy storage devices from the same manufacturing batch. These varying leakage rates result in the voltage across different energy storage devices decreasing at different rates. When the energy storage devices 102, 104 are charged, the energy storage device with the lower leakage rate, and hence the greater voltage, can exceed the maximum voltage specified for that energy storage device before the combined voltage of both energy storage devices reaches a desired terminal voltage. Resistors 106, 108 are placed in parallel with energy storage devices 102, 104 respectively in order to equalise the respective voltage drops across each energy storage device. Charging leads 110 are shown in the drawing, for connecting a charging circuit (not shown) to the energy storage devices.
As one skilled in the art can appreciate, the resistors act to increase the total current flowing through each energy storage device, since the resistors are effectively in parallel with the parasitic resistance of the energy storage devices. This causes the energy storage devices to discharge any excess charge faster than if the resistors were not present. The resistor values are normally chosen so that the current in each resistor is much greater than the largest specified internal leakage current of the individual energy storage device. Given that the resistors typically come from the same manufacturing batch and are quite closely matched in value (within a few percent), the rate at which the voltage of the energy storage devices decrease is therefore more closely matched than if the resistors were absent.
However, this configuration results in continually wasted power since current is constantly flowing through the resistors and the current in each resistor is greater than the leakage current of the capacitor. A more power-efficient solution is required.
It is therefore desirable to provide a configuration that allows current to flow only when an energy storage component is above a predetermined voltage and thereby conserve power.